sync

package standard library
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Published: Jun 1, 2022 License: BSD-3-Clause Imports: 4 Imported by: 1,110,209

Documentation

Overview

Package sync provides basic synchronization primitives such as mutual exclusion locks. Other than the Once and WaitGroup types, most are intended for use by low-level library routines. Higher-level synchronization is better done via channels and communication.

Values containing the types defined in this package should not be copied.

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type Cond

type Cond struct {

	// L is held while observing or changing the condition
	L Locker
	// contains filtered or unexported fields
}

Cond implements a condition variable, a rendezvous point for goroutines waiting for or announcing the occurrence of an event.

Each Cond has an associated Locker L (often a *Mutex or *RWMutex), which must be held when changing the condition and when calling the Wait method.

A Cond must not be copied after first use.

func NewCond

func NewCond(l Locker) *Cond

NewCond returns a new Cond with Locker l.

func (*Cond) Broadcast

func (c *Cond) Broadcast()

Broadcast wakes all goroutines waiting on c.

It is allowed but not required for the caller to hold c.L during the call.

func (*Cond) Signal

func (c *Cond) Signal()

Signal wakes one goroutine waiting on c, if there is any.

It is allowed but not required for the caller to hold c.L during the call.

func (*Cond) Wait

func (c *Cond) Wait()

Wait atomically unlocks c.L and suspends execution of the calling goroutine. After later resuming execution, Wait locks c.L before returning. Unlike in other systems, Wait cannot return unless awoken by Broadcast or Signal.

Because c.L is not locked when Wait first resumes, the caller typically cannot assume that the condition is true when Wait returns. Instead, the caller should Wait in a loop:

c.L.Lock()
for !condition() {
    c.Wait()
}
... make use of condition ...
c.L.Unlock()

type Locker

type Locker interface {
	Lock()
	Unlock()
}

A Locker represents an object that can be locked and unlocked.

type Map added in go1.9

type Map struct {
	// contains filtered or unexported fields
}

Map is like a Go map[interface{}]interface{} but is safe for concurrent use by multiple goroutines without additional locking or coordination. Loads, stores, and deletes run in amortized constant time.

The Map type is specialized. Most code should use a plain Go map instead, with separate locking or coordination, for better type safety and to make it easier to maintain other invariants along with the map content.

The Map type is optimized for two common use cases: (1) when the entry for a given key is only ever written once but read many times, as in caches that only grow, or (2) when multiple goroutines read, write, and overwrite entries for disjoint sets of keys. In these two cases, use of a Map may significantly reduce lock contention compared to a Go map paired with a separate Mutex or RWMutex.

The zero Map is empty and ready for use. A Map must not be copied after first use.

func (*Map) Delete added in go1.9

func (m *Map) Delete(key any)

Delete deletes the value for a key.

func (*Map) Load added in go1.9

func (m *Map) Load(key any) (value any, ok bool)

Load returns the value stored in the map for a key, or nil if no value is present. The ok result indicates whether value was found in the map.

func (*Map) LoadAndDelete added in go1.15

func (m *Map) LoadAndDelete(key any) (value any, loaded bool)

LoadAndDelete deletes the value for a key, returning the previous value if any. The loaded result reports whether the key was present.

func (*Map) LoadOrStore added in go1.9

func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool)

LoadOrStore returns the existing value for the key if present. Otherwise, it stores and returns the given value. The loaded result is true if the value was loaded, false if stored.

func (*Map) Range added in go1.9

func (m *Map) Range(f func(key, value any) bool)

Range calls f sequentially for each key and value present in the map. If f returns false, range stops the iteration.

Range does not necessarily correspond to any consistent snapshot of the Map's contents: no key will be visited more than once, but if the value for any key is stored or deleted concurrently (including by f), Range may reflect any mapping for that key from any point during the Range call. Range does not block other methods on the receiver; even f itself may call any method on m.

Range may be O(N) with the number of elements in the map even if f returns false after a constant number of calls.

func (*Map) Store added in go1.9

func (m *Map) Store(key, value any)

Store sets the value for a key.

type Mutex

type Mutex struct {
	// contains filtered or unexported fields
}

A Mutex is a mutual exclusion lock. The zero value for a Mutex is an unlocked mutex.

A Mutex must not be copied after first use.

func (*Mutex) Lock

func (m *Mutex) Lock()

Lock locks m. If the lock is already in use, the calling goroutine blocks until the mutex is available.

func (*Mutex) TryLock added in go1.18

func (m *Mutex) TryLock() bool

TryLock tries to lock m and reports whether it succeeded.

Note that while correct uses of TryLock do exist, they are rare, and use of TryLock is often a sign of a deeper problem in a particular use of mutexes.

func (*Mutex) Unlock

func (m *Mutex) Unlock()

Unlock unlocks m. It is a run-time error if m is not locked on entry to Unlock.

A locked Mutex is not associated with a particular goroutine. It is allowed for one goroutine to lock a Mutex and then arrange for another goroutine to unlock it.

type Once

type Once struct {
	// contains filtered or unexported fields
}

Once is an object that will perform exactly one action.

A Once must not be copied after first use.

Example
package main

import (
	"fmt"
	"sync"
)

func main() {
	var once sync.Once
	onceBody := func() {
		fmt.Println("Only once")
	}
	done := make(chan bool)
	for i := 0; i < 10; i++ {
		go func() {
			once.Do(onceBody)
			done <- true
		}()
	}
	for i := 0; i < 10; i++ {
		<-done
	}
}
Output:

Only once

func (*Once) Do

func (o *Once) Do(f func())

Do calls the function f if and only if Do is being called for the first time for this instance of Once. In other words, given

var once Once

if once.Do(f) is called multiple times, only the first call will invoke f, even if f has a different value in each invocation. A new instance of Once is required for each function to execute.

Do is intended for initialization that must be run exactly once. Since f is niladic, it may be necessary to use a function literal to capture the arguments to a function to be invoked by Do:

config.once.Do(func() { config.init(filename) })

Because no call to Do returns until the one call to f returns, if f causes Do to be called, it will deadlock.

If f panics, Do considers it to have returned; future calls of Do return without calling f.

type Pool added in go1.3

type Pool struct {

	// New optionally specifies a function to generate
	// a value when Get would otherwise return nil.
	// It may not be changed concurrently with calls to Get.
	New func() any
	// contains filtered or unexported fields
}

A Pool is a set of temporary objects that may be individually saved and retrieved.

Any item stored in the Pool may be removed automatically at any time without notification. If the Pool holds the only reference when this happens, the item might be deallocated.

A Pool is safe for use by multiple goroutines simultaneously.

Pool's purpose is to cache allocated but unused items for later reuse, relieving pressure on the garbage collector. That is, it makes it easy to build efficient, thread-safe free lists. However, it is not suitable for all free lists.

An appropriate use of a Pool is to manage a group of temporary items silently shared among and potentially reused by concurrent independent clients of a package. Pool provides a way to amortize allocation overhead across many clients.

An example of good use of a Pool is in the fmt package, which maintains a dynamically-sized store of temporary output buffers. The store scales under load (when many goroutines are actively printing) and shrinks when quiescent.

On the other hand, a free list maintained as part of a short-lived object is not a suitable use for a Pool, since the overhead does not amortize well in that scenario. It is more efficient to have such objects implement their own free list.

A Pool must not be copied after first use.

Example
package main

import (
	"bytes"
	"io"
	"os"
	"sync"
	"time"
)

var bufPool = sync.Pool{
	New: func() any {
		// The Pool's New function should generally only return pointer
		// types, since a pointer can be put into the return interface
		// value without an allocation:
		return new(bytes.Buffer)
	},
}

// timeNow is a fake version of time.Now for tests.
func timeNow() time.Time {
	return time.Unix(1136214245, 0)
}

func Log(w io.Writer, key, val string) {
	b := bufPool.Get().(*bytes.Buffer)
	b.Reset()
	// Replace this with time.Now() in a real logger.
	b.WriteString(timeNow().UTC().Format(time.RFC3339))
	b.WriteByte(' ')
	b.WriteString(key)
	b.WriteByte('=')
	b.WriteString(val)
	w.Write(b.Bytes())
	bufPool.Put(b)
}

func main() {
	Log(os.Stdout, "path", "/search?q=flowers")
}
Output:

2006-01-02T15:04:05Z path=/search?q=flowers

func (*Pool) Get added in go1.3

func (p *Pool) Get() any

Get selects an arbitrary item from the Pool, removes it from the Pool, and returns it to the caller. Get may choose to ignore the pool and treat it as empty. Callers should not assume any relation between values passed to Put and the values returned by Get.

If Get would otherwise return nil and p.New is non-nil, Get returns the result of calling p.New.

func (*Pool) Put added in go1.3

func (p *Pool) Put(x any)

Put adds x to the pool.

type RWMutex

type RWMutex struct {
	// contains filtered or unexported fields
}

A RWMutex is a reader/writer mutual exclusion lock. The lock can be held by an arbitrary number of readers or a single writer. The zero value for a RWMutex is an unlocked mutex.

A RWMutex must not be copied after first use.

If a goroutine holds a RWMutex for reading and another goroutine might call Lock, no goroutine should expect to be able to acquire a read lock until the initial read lock is released. In particular, this prohibits recursive read locking. This is to ensure that the lock eventually becomes available; a blocked Lock call excludes new readers from acquiring the lock.

func (*RWMutex) Lock

func (rw *RWMutex) Lock()

Lock locks rw for writing. If the lock is already locked for reading or writing, Lock blocks until the lock is available.

func (*RWMutex) RLock

func (rw *RWMutex) RLock()

RLock locks rw for reading.

It should not be used for recursive read locking; a blocked Lock call excludes new readers from acquiring the lock. See the documentation on the RWMutex type.

func (*RWMutex) RLocker

func (rw *RWMutex) RLocker() Locker

RLocker returns a Locker interface that implements the Lock and Unlock methods by calling rw.RLock and rw.RUnlock.

func (*RWMutex) RUnlock

func (rw *RWMutex) RUnlock()

RUnlock undoes a single RLock call; it does not affect other simultaneous readers. It is a run-time error if rw is not locked for reading on entry to RUnlock.

func (*RWMutex) TryLock added in go1.18

func (rw *RWMutex) TryLock() bool

TryLock tries to lock rw for writing and reports whether it succeeded.

Note that while correct uses of TryLock do exist, they are rare, and use of TryLock is often a sign of a deeper problem in a particular use of mutexes.

func (*RWMutex) TryRLock added in go1.18

func (rw *RWMutex) TryRLock() bool

TryRLock tries to lock rw for reading and reports whether it succeeded.

Note that while correct uses of TryRLock do exist, they are rare, and use of TryRLock is often a sign of a deeper problem in a particular use of mutexes.

func (*RWMutex) Unlock

func (rw *RWMutex) Unlock()

Unlock unlocks rw for writing. It is a run-time error if rw is not locked for writing on entry to Unlock.

As with Mutexes, a locked RWMutex is not associated with a particular goroutine. One goroutine may RLock (Lock) a RWMutex and then arrange for another goroutine to RUnlock (Unlock) it.

type WaitGroup

type WaitGroup struct {
	// contains filtered or unexported fields
}

A WaitGroup waits for a collection of goroutines to finish. The main goroutine calls Add to set the number of goroutines to wait for. Then each of the goroutines runs and calls Done when finished. At the same time, Wait can be used to block until all goroutines have finished.

A WaitGroup must not be copied after first use.

Example

This example fetches several URLs concurrently, using a WaitGroup to block until all the fetches are complete.

package main

import (
	"sync"
)

type httpPkg struct{}

func (httpPkg) Get(url string) {}

var http httpPkg

func main() {
	var wg sync.WaitGroup
	var urls = []string{
		"http://www.golang.org/",
		"http://www.google.com/",
		"http://www.example.com/",
	}
	for _, url := range urls {
		// Increment the WaitGroup counter.
		wg.Add(1)
		// Launch a goroutine to fetch the URL.
		go func(url string) {
			// Decrement the counter when the goroutine completes.
			defer wg.Done()
			// Fetch the URL.
			http.Get(url)
		}(url)
	}
	// Wait for all HTTP fetches to complete.
	wg.Wait()
}
Output:

func (*WaitGroup) Add

func (wg *WaitGroup) Add(delta int)

Add adds delta, which may be negative, to the WaitGroup counter. If the counter becomes zero, all goroutines blocked on Wait are released. If the counter goes negative, Add panics.

Note that calls with a positive delta that occur when the counter is zero must happen before a Wait. Calls with a negative delta, or calls with a positive delta that start when the counter is greater than zero, may happen at any time. Typically this means the calls to Add should execute before the statement creating the goroutine or other event to be waited for. If a WaitGroup is reused to wait for several independent sets of events, new Add calls must happen after all previous Wait calls have returned. See the WaitGroup example.

func (*WaitGroup) Done

func (wg *WaitGroup) Done()

Done decrements the WaitGroup counter by one.

func (*WaitGroup) Wait

func (wg *WaitGroup) Wait()

Wait blocks until the WaitGroup counter is zero.

Directories

Path Synopsis
Package atomic provides low-level atomic memory primitives useful for implementing synchronization algorithms.
Package atomic provides low-level atomic memory primitives useful for implementing synchronization algorithms.

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